gcc: xtensa: fix __builtin_frame_address use with shared libgcc

The xtensa port uses __xtensa_libgcc_window_spill in libgcc to implement
__builtin_frame_address. This symbol is local/hidden in libgcc. This is not a
problem when linking against static libgcc. But g++ defaults to
-shared-libgcc, thus breaking link against C++ shared libraries that are using
__builtin_frame_address as follows:

ld: test: hidden symbol `__xtensa_libgcc_window_spill' in .../libgcc.a(lib2funcs.o) is referenced by DSO

Add upstream patches that make libgcc_s.so a linker script that links in
unresolved symbols from the static libgcc, similar to the ARM and PowerPC
ports.

Fixes:
http://autobuild.buildroot.net/results/e2d/e2d1a763fa86b8575e2e48e6d73c018175f43e7c/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>